1. Technical Field of the Invention
The present invention concerns a process for producing a tire having several mixtures comprising, as the majority filler, a non-conductive filler such as silica or mixtures filled with small proportions of carbon black, at least two of these mixtures constituting the tire tread. The invention also relates to apparatus for implementing such a process.
2. The Related Art
As environmental problems become more and more crucial, fuel economy and ways to combat the nuisances engendered by motor vehicles having become a priority, one of the objective of tire manufacturers is to produce a tire which combines very low rolling resistance, excellent grip on dry, wet, snowy or icy ground, very good wear resistance, and finally, low rolling noise.
To achieve that objective, European Patent Application EP A 501 227 (U.S. Pat. No. 5,227,425) proposes a tire having a tread that comprises silica as its main reinforcing filler. Although this solution gives the best compromise between the various very contradictory properties mentioned above, it has nevertheless been found that, depending on the vehicles, tires with a tread comprising silica as the main reinforcing filler have the disadvantage of being able to accumulate more or less high levels of static electricity formed by friction of the tire against the road while the vehicle is driving, since silica is not electrically conductive.
When certain particular conditions coexist, the static electricity so accumulated in a tire can give rise to a disagreeable electric shock for the occupant of the vehicle when he touches the body of the vehicle. Furthermore, this static electricity can hasten the ageing of the tire because of the ozone produced by the electric discharge. Depending on the nature of the ground and the vehicle, it can also cause the on-board radio to malfunction because of the interference it generates.
The problem of static electricity accumulation in a tire, and most of the disadvantages related to it, has been familiar for a very long time and had already arisen when the reinforcing filler used was carbon black.
Application EP 0 658 452 A1 describes the adaptation of long-known principles to a tire said to be modern, an adaptation that solves the main problems associated with the solutions proposed in various earlier documents and, in particular, the harmful heterogeneities introduced into the structures of tires. The solution proposed consists in inserting a strip of conductive rubber mixture or insert, preferentially extending all round the circumference of the tire and connecting the surface of the tread either to one of the crown plies, or to the carcass reinforcement, or to any other part of the tire that is a sufficiently good conductor of electricity, the necessary electrical conductivity being conferred by the presence of a suitable carbon black.
Although such a solution is perfectly viable for a tire having a tread that consists of a single non-conductive mixture, for example a tread of a passenger car tire, this is not true in the case of a tire having several layers of rubber mixtures above the carcass reinforcement and layers of rubber mixtures between the crown reinforcement and the carcass reinforcement, as is the case in any tire designed to roll with a high steady-state operating temperature, as in heavy vehicles or high-speed vehicles.
In effect, if for any reason such a tire has been provided with a non-conductive layer or internal portion of the tread (the part not in contact with the ground) between the crown reinforcement and the external portion of the tread (the part in contact with the ground) that has been made conductive by the presence of a circumferential or strip-shaped insert, the said internal portion must be made conductive. Similarly, a layer between the carcass reinforcement and the crown reinforcement, which produces the well known excess thickness in the edge areas of the crown plies, must also be made conductive if it is not so to begin with.
As described in the applicants' French application FR 97/02276 (WO 98/38050), a first solution is to ensure electrical connection between two first layers that are conductive or have been made so and are separated by a third, non-conductive layer, by means of at least one strip of rubber mixture having small thickness, width and length, positioned between the two joint faces of the third, non-conductive layer and in contact with the means that make the first two layers conductive, such that they are connected thereby. Although industrially satisfactory, this method entails the positioning of a supplementary product and leads to additional manufacturing cost.
As described in the applicants' international application WO 99/43506 (U.S. application Ser. No. 09/645,097, filed Feb. 17, 1999), a second solution consists in providing each non-conductive layer with a circumferential insert of conductive mixture after the nonconductive layer has been extruded by the usual extrusion means, and then joining the two products together before they are positioned on the crown, the said layers having a common contact surface and the width of the insert in one layer at the level of the contact surface being equal to at least 10 times that of the insert in the other layer at the same level. This method effectively enables each layer to be made electrically conductive and ensures electrical connection between them and the carcass reinforcement.
However, the method becomes too complex when there are no longer only two layers consisting of two distinct rubber mixtures, but three layers, four layers or more, with different rubber mixtures.
Besides, certain rubber mixtures have rheological properties so different that bonding them together is very difficult. This in particular is the case when one of the mixtures is more decohesive than the other. When it is desired to join such mixtures, for example within a tread, and to avoid artifices such as using a bonding agent, such as a specific glue or adhesive, to join the two, the method used is coextrusion which, moreover, has a certain economic advantage in industry. In effect, in a coextrusion process the different products, which have been worked separately by extrusion screws, are propelled towards a common extrusion orifice that allows the products to be brought together while hot and under pressure.
In this configuration, it could be envisaged to transpose the second solution to coextrusion. Thus, an insert would be produced in each product upstream from the extrusion orifice by means of two micro-extruders, the nozzle of each micro-extruder passing through each product so that, in conformity with the second solution, one of the inserts has a base at least ten times as wide as the base of the second product opposite it. However, on the one hand, such a solution considerably increases the bulk of the coextrusion machine, and on the other hand, it greatly adds to the difficulty of the operations of changing the extruded product and cleaning the extruders.